This invention relates to a method of copolymerizing ethylene with cycloolefin monomers and the homogeneous addition copolymers obtained thereby. More particularly, this invention is directed to a copolymerization process which provides uniform ethylene/cycloolefin copolymer products with consistent property profiles. The cycloolefin monomers used herein are those referred to as norbornene-type monomers and are described more particularly below.
Methods for producing copolymers of ethylene and norbornene-type monomers are well known. Polymerization can proceed by either a ring-opening mechanism or by an addition reaction wherein the cyclic ring structure remains intact. Ring-opening polymerizations are discussed with greater particularity in U.S. Pat. Nos. 4,136,247 and 4,178,424, assigned to the same assignee as the present invention and are incorporated herein by reference for their discussion of such polymerizations. Ring-opening polymerization generally yields unsaturated linear polymers while addition polymerization yields polycycloaliphatics. In both polymerization schemes, it is desirable to produce polymers having high molecular weight monomers incorporated therein to provide good temperature resistance, i.e., high heat distortion temperatures.
To maintain either mechanism exclusive, special process conditions are utilized in each polymerization method. Different temperature ranges and catalysts are preferred for each polymerization method, respectively. It is emphasized this invention is directed to addition copolymers of ethylene and norbornene-type monomers and methods for making these addition copolymers.
Copolymers of ethylene and bicycloheptene, a norbornene-type monomer defined below, are described in U.S. Pat. No. 2,799,668 by Anderson. Anderson discloses the ratio of ethylene to bicycloheptene in the copolymers obtained can vary from small quantities of ethylene to small quantities of bicycloheptene. However, Anderson does not show or suggest addition polymerization is exclusive in the process disclosed. In addition, Anderson was unsuccessful in obtaining useful copolymers when incorporating large quantities of bicycloheptene in that these copolymers were brittle where the concentration of bicycloheptene exceeded 50 mole percent.
Stamatoff, U.S. Pat. No. 2,883,372, also discloses copolymers of ethylene and the norbornene-type monomer, dihydrodicyclopentadiene. As with Anderson, there is no indication the addition polymerization reaction is exclusive and incorporating large quantities of the norbornene-type monomer provided brittle copolymers with low glass transition temperatures.
U.S. Pat. No. 3,494,897, Reding et al., alleges in its disclosure to provide the first production of addition polymerized copolymers of ethylene and bicyclo(2.2.1)hept-2-ene monomers wherein no ring-opening polymerization takes place in producing the copolymer. The bicyclo(2.2.1)hept-2-ene monomers fall within the scope of the norbornene-type monomers defined below. To obtain these polymers, Reding et al. discloses the use of a free radical catalyst at a temperature above 40.degree. C. and pressures of ethylene gas of at least 500 atmospheres. This patent does not provide methods wherein high concentrations of the bicyclo(2.2.1)hept-2-ene monomer can be incorporated into the copolymer in a uniform manner. The copolymers obtained in the Examples generally have less than 10 weight percent of the high molecular weight norbornene-type monomer. Although one Example demonstrates up to 60 weight percent of the norbornene-type monomer within the copolymer, this translates to a mole ratio of norbornene-type monomer to ethylene of less than 0.5. It is desirable to incorporate higher concentrations of norbornene-type monomer into the copolymer with .alpha.-olefin.
European Patent Application No. 156464, Kajiura et al., does incorporate larger quantities of norbornene-type monomers into addition copolymers with ethylene. The addition copolymers disclosed by Kajiura et al. comprise ethylene and tetracyclododecene derivatives and other norbornene-type monomers. Kajiura et al. indicate the quantity of norbornene-type monomer within the copolymer is greater than 3 mole percent and as much as 95 mole percent of the copolymer, thereby obtaining mole ratios of norbornene-type monomer to ethylene greater than 1. These copolymers are obtained by reaction of the norbornene-type monomer and ethylene within a hydrocarbon solvent in the presence of a catalyst. Suitable catalysts comprise vanadium compounds in combination with organoaluminum compounds which are soluble in the hydrocarbon solvent. The use of large quantities of solvent is disclosed in the Examples provided by Kajiura et al. For example, volumes of 1 liter and 250 milliliters of solvent are used for about 30 to 40 grams of norbornene-type monomer. This translates to a volume ratio of solvent to norbornene-type monomer of about 25:1, where 1 liter of solution is used and a volume ratio of about 8:1, where 250 milliliters of solution are utilized.
The process of Kajiura et al. produces random addition copolymers of ethylene and the norbornene-type monomer. With the higher concentrations of norbornene-type monomer incorporated in the copolymer, higher glass transition temperatures are obtained. However, these random copolymers suffer in that the glass transition temperatures of the final products vary with minor process variations, such as the percent yield of copolymer, the concentration of cycloolefin monomer in the reaction medium and the concentration of ethylene in the reaction medium. Such variations in the property profile, particularly the glass transition temperature, is unacceptable for most commercial applications of an engineering polymer.
It is desirable to obtain a more homogenous addition copolymer of ethylene and norbornene-type monomer with compositional uniformity so as to provide consistent glass transition temperatures and good optical properties without obtaining crystallinity. A more structured copolymer, such as an alternating copolymer of norbornene-type monomer and ethylene, should not exhibit variations in engineering properties with variations in reactant concentrations. However, prior to this invention, homogeneous addition copolymers of norbornene-type monomers and ethylene, and methods for producing the same, were not available.